In the typical production of low density polyethylene, a reactor discharges a stream which is a mixture of polymer and unreacted materials to a product receiver. The product receiver operates at a pressure substantially below the reactor pressure and flow of the reactor discharge is controlled by the product valve. In the product receiver, the major portion of the unreacted materials are removed due to flashing which results from the drop in pressure experienced by the mixture. The flashed material, commonly referred to as the return gas, is subsequently returned to the reactor. The remaining polymerized material settles in the product receiver and still contains some unreacted materials which are removed in the remainder of the polymer recovery system.
The polymer discharged from the product receiver is fed to an extruder through a polymer flow control system. The extruder performs two functions in this system: (1) final devolatilization to remove the remaining unreacted material; (2) pumping of the polymer through a screen pack, if one is being used, and a pelletizer die plate.
The material enters the side of the extruder and the unreacted materials flash and form a foam having a very low density. Therefore an extruder having a very large volumetric conveying capacity in the feed section is necessary to handle the material as the final devolatilization is occurring. Normally, an extruder having a two-diameter screw or an oversize single-diameter screw is necessary to obtain the necessary conveying capacity to handle the material entering the extruder. In some installations a portion of the flashed material is removed from the extruder through a top mounted vent stack.
As the production rate of single low density polyethylene (LDPE) reactors are increased, larger and larger extruders, which become prohibitively expensive, are needed. In an effort to eliminate the use of two-diameter extruders or oversized extruders, some existing units have been modified to include a secondary ethylene separation (flashing) operation upstream of the extruder inlet subsequent to the primary product receiver ethylene separation (flashing) operation.
This system differs from the side fed extruder top mounted vent stack type in that the material is fed into the top of the vent stack and essentially all of the remaining unreacted materials are released before the polymer stream enters the extruder. This provides a material to the extruder which has a much greater density and eliminates the need for two-diameter extruders or large single-diameter extruders. The devolatilization and pumping functions of the original, two-diameter extruder system have now been separated, i.e. the final devolatilization is performed in the vent stack and only the polymer pumping is performed by the extruder. However, extruders pump polymer by developing viscous drag, and are very inefficient pumps.